Due to their physical and chemical properties, such as high resistance to chemical attack and good adhesion to various substrates, epoxy resins are useful in the preparation of powder coatings. Conventionally, an epoxy powder coating binder system is prepared by blending an epoxy resin with a coreactant, such as a compound that contains either one or more reactive phenolic hydroxyl groups, or one or more reactive amine groups that are capable of reacting with the epoxide groups to form a hard, infusible coating. This epoxy powder coating binder system can then be combined with other additives such as, additional curing agents, pigments, flow control agents, etc. to form a suitable epoxy powder coating composition for coating metallic substrates.
Generally, the adhesion of epoxy powder coating compositions to the substrate is adequate. However, adhesion of the presently available epoxy powder coating compositions to metallic substrates under hot and humid conditions continues to be a problem. This is especially true of the epoxy powder coating compositions presently available for coating rebars and the interior and exterior of pipes.
U.S. Pat. No. 4,678,712 to Elliot and U.S. Pat. No. 4,330,644 to Allen disclose various rebar and pipe epoxy powder coating compositions that have the epoxy resin pre-reacted with a hydroxylamine to form an epoxy-amine adduct prior to being added to the powder pre-mix. However, such powder coatings still suffer from poor humidity resistance.
Epoxy powder coatings have also been used in the past on gas and oil pipelines to prevent corrosion, as well as, facilitate cathodic protection of the pipe. Cathodic protection is another means for preventing corrosion of iron containing metallic materials, such as steel in humid conditions containing electrolytes, i.e., brine and salt solutions. In general, cathodic protection prevents dissolution of the iron containing metallic material by maintaining the material as a cathode and inhibiting ionization of the iron contained therein. The iron containing metallic material, however, is generally not used by itself to provide cathodic protection because when the iron portion has a large area the consumption of power and a sacrificial anode increases. Instead, cathodic protection is generally effected by applying an organic coating and/or lining to the iron containing metallic material. Through this approach, the majority of the iron containing metallic material is protected from corrosion, and any corrosion that might arise as a result of defective portions occurring in the organic coating and/or lining, such as scratches and/or pin-holes, can be supplementally prevented through cathodic protection.
Unfortunately, it is extremely difficult to predict the exact size of the surface area at risk, and therefore excessive amounts of power and cathodic protection end up being applied to the iron containing metallic material. When excessive cathodic protection is applied, however, there is excessive polarization, which causes hydroxyl ions to be generated via hydrolysis of water at the cathode. As a result, the metal exposed at the scratched portions of the organic coating ends up functioning as a cathode, and the organic coating is therefore always exposed to an alkaline environment. Eventually, these conditions cause the points of adhesion of the organic coating and/or lining to degrade at the interface between the metallic material and the organic coating, as well as, between the organic coatings, particularly at the points where alkali resistance is weakest. As a result, cathodic disbonding of the organic coating occurs.
As a means for restricting such cathodic disbanding, Japanese Unexamined Patent Publication (Kokai) No. 59-222275 proposes using either a chromate treatment method, or a zinc-rich primer coating of a specific thermosetting epoxide resin, and Japanese Unexamined Patent Publication (Kokai) No. 55-142063 proposes using a composition consisting of a polyvinyl butyral resin, a liquid epoxide resin, a borate compound, an epoxy-silane coupling agent and phosphoric acid as a pre-treatment composition for baking type.
In addition, European Patent No. 0 588 318 B1 to Kaga mentions a method for providing cathodic protection that involves using steel pre-treatment steps, applying a thermosetting epoxide resin based powder coating containing 5 to 75 wt. % zinc compounds, and subsequently polarizing the coated steel material as a cathode. Using high levels of zinc compounds, however, presents issues of solubility over long periods of time, as well as, as increased costs due to the high price of zinc borate compounds.
Accordingly, there is a need for powder coating compositions, and methods of application thereof, that provide optimum short and long term high temperature and humidity cathodic disbondment protection at a lower cost. There is also a need for powder coating compositions, and methods of application thereof, that provide improved adhesion to a substrate under hot and humid conditions, which can be applied at lower temperatures, and therefore at lower energy consumption costs.